An oil-impregnated sintered bearing which is made of a porous sintered metal and impregnates lubricating oil can be used for a long time without refueling, is excellent in resistance at a high temperature, and generates low noise. Therefore, it is broadly used as a bearing for a rotary shaft to replace a ball bearing.
In this kind of oil-impregnated sintered bearing, a bearing hole is provided in a bearing body made of a porous sintered alloy, wherein a rotary shaft with a smaller diameter than the bearing hole is inserted in the bearing hole. In the oil-impregnated sintered bearing, lubricating oil extracted from a large number of small vacancies in the bearing body by the pumping action attributed to the rotation of the rotary shaft, and oozed lubricating oil due to the expansion attributed to frictional heat forms an oil film on a sliding part with the rotary shaft. Owing to the oil film, the sintered oil-retained bearing can support the rotary shaft without causing problems such as a seizure.
In these kinds of oil-impregnated sintered bearing, a large number of vacancies which impregnate lubricating oil are formed even on the sliding surface with which the rotary shaft is slidingly-contacted. Therefore, as described above, even if the oil film is formed between the rotary shaft and sliding surface, oil pressure is decreased, because a part of the lubricating oil is leaked from the vacancies, and thus it is easy to generate local contact of the rotary shaft with the sliding surface. Therefore, these kinds of oil-impregnated sintered bearing has a defect such that the frictional coefficient to the rotary shaft becomes larger and a seizure is easily generated.
So, there is suggested an oil-impregnated sintered bearing, including a bearing body which is made of a porous sintered alloy containing vacancies and has a bearing hole in which a rotary shaft can be inserted, and a sliding surface, which is an area where the vacancies on an inner peripheral surface of the bearing hole are crushed (compressed, flattened, depressed) (for example, refer to Japanese Unexamined Patent Application Publication No. 2002-122142). In such an oil-impregnated sintered bearing, a rough part formed on the outer peripheral surface of a core rod crushes the vacancies, and thus lubricating oil is not impregnated from the sliding surface into the bearing body, and the oil film formed on the sliding surface is retained.
On the other hand, in such an oil-impregnated sintered bearing, because a large number of vacancies are formed not only on a sliding surface, with which a rotary shaft is slidingly-contacted, but are also formed on an outer peripheral surface, there is a problem in that, if the outer peripheral surface of the oil-impregnated sintered bearing is exposed when used, a part of the lubricating oil is leaked from the vacancies on the outer peripheral surface, and oil pressure becomes decreased.
Therefore, in order to prevent the leakage of the lubricating oil from the outer peripheral surface, a cylindrical attachment is provided at a used position, and the bearing is equipped in the attachment. Also in order to prevent the leakage of the lubricating oil, the outer peripheral surface of the bearing is covered with a covering material such as felt.